The principal cells of mammalian cortical collecting tubules (CCT) are the major site for regulation of total body Na+ and water balance. Na+ ions enter these specialized cells through amiloride-sensitive, highly selective protein pores in the apical membrane. This is driven by the electrochemical gradient created by the basolateral Na+/K+ATPase. Under virtually all conditions the control of total body sodium is determined by the regulation of the apical sodium channels. Historically, the control of sodium channels was generally attributed to regulation by systemic hormones reaching the basolateral surface of CCT cells via the peritubular circulation. However, it is also clear that, besides circulating hormones, there are intrarenal control mechanisms which depend upon delivery of hormones in the tubular fluid to receptors ont he apical surface of distal nephron cells. The source of these hormones is either through filtration and concentration of systemic hormones or the local production and secretion of the hormones within the kidney. These hormone receptors appear to interact with guanine nucleotide binding proteins which subsequently activate signal transduction pathways confined to athe apical pole of the epithelial cells and which regulate apical sodium channel activity. Thus, there are two separate control mechanisms linked by transcellular messenger. Intrarenal production of hormones would provide an exquisitely sensitive mechanism for adjusting Na+ reabsorption in response to changes in renal perfusion, filtration, or function. The specific aims of th proposal are to (1) examine the effects of three putative luminal hormones (angiotensin II, vasopressin, and dopamine) on apical Na+ channels in cultured A6 distal nephron cells. (2) examine the intracellular signaling pathways activated by luminal hormones in cultured distal nephron cells with special emphasis on pathways already known to affect apical Na+ channel activity. Specifically, examine the linkage of luminal receptors to apical guanine nucleotide-binding proteins (G proteins); examine the linkage of G proteins to three signal transduction pathways which could modify sodium channel activity including, first, adenylyl cyclase activity, intracellular cyclic AMP, and cAMP-dependent protein kinase A; second, phospholipase C, inositol polyphosphates, diacylglycerol, intracellular Ca2+, and protein kinase C isoenzymes; and third, particularly for angiotensin II, determine if there is induction of tyrosine kinase activity.